Generally, the present invention relates to enhanced oil recovery by employing chemical floods containing a surfactant, and more particularly to an improvement in the method of chemical flooding wherein a sacrificial adsorbate is utilized to reduce the adsorption of the surfactant in the matrix of the oil-containing subterranean earth formation.
The recovery of oil from oil-bearing subterranean earth formations, or reservoirs, is usually initially achieved by employing primary recovery procedures which entail the use of pumping mechanisms together with natural reservoir pressures. Often the reservoir pressure is sufficient to recover a considerable quantity of oil prior to the initiation of pumping.
Enhanced oil recovery procedures are used for extracting additional oil from the reservoir subsequent to the primary recovery operations. One of the most common enhanced oil recovery procedures is the use of a water flood where water is injected under pressure into the subterranean earth formation to displace the oil in the formation towards a suitable production well for the recovery thereof. Inasmuch as water is immiscible with oil and results in high interfacial tension therebetween, a significant amount of the "recoverable" oil remains in the reservoir. Chemical flooding procedures are then utilized to displace additional oil from the reservoir. In chemical flooding a slug of water-based emulsion containing an organic sulfonate surfactant and alcohol co-surfactants is injected into the well for lowering the interfacial tension between the oil and water and thereby providing a more efficient oil recovery mechanism. Often the surfactant emulsion is driven through the oil reservoir by a pusher formed of a "thick" water which is thickened by a polyacrylamide or by a polymer. The combined sulfonate emulsion and pusher is commonly referred to as micellar flooding. The reduced surface tension caused by the emulsion permits oil in the pores of the rock to mix more readily with the surfactant emulsion so that oil is forced out of the reservoir and coalesces as the slug is pushed along. In the use of such chemical floods, a significant percentage of the surfactant is adsorbed in the formation matrix so as to significantly alter the composition of the flood as well as to significantly reduce the amount of surfactant as it progresses through the formation.
The adsorption of the surfactant component in the formation matrix reduces the effectiveness of the chemical flood as it progresses through the earth formation. To over-come this adsorption problem, additional surfactants must be included in the flood to minimize the deleterious effects of the surfactant adsorption. Other efforts to minimize the effects of adsorption of the surfactant include the use of competitive or sacrificial adsorbates which are injected into the earth formation prior to or concurrent with the surfactant dispersion. These sacrifical adsorbates compete with the surfactant and adsorb on potential adsorption sites in the formation matrix to thereby significantly diminish the overall surfactant adsorption as the emulsion progresses through the earth formation. A group of chemical compounds found to possess sacrificial adsorbate properties is lignosulfonates, such as described in U.S. Pat. No. 4,006,779 which issued Feb. 8, 1977 and entitled "Lignosulfonates As Sacrificial Agents in Oil Recovery Processes". A further discussion of lignosulfonate is in the research publication article entitled "Wood Chemicals Inject New Life into Tired Oil Wells," June 20, 1979, of The Chemical Week, p. 65.
A suitable sacrifical adsorbate should be cost-effective in chemical floods since it is to be sacrificed or adsorbed by the formation and cannot be readily recovered. The lignosulfonates are relatively inexpensive as compared to sulfonates used in chemical floods. Lignosulfonates are by-products of a wood pulping process utilizing sulfites or as a product of the sulfonation of lignin fraction recovered from the waste streams of wood pulping processes including the common kraft process. This provides a relatively abundant source of lignin which is the basic constituent utilized in manufacturing lignosulfonates. There are, however, several different uses of lignosulfonates, such as in the manufacture of drilling mud and glues suitable for laminated woods which increase the cost of lignosulfonates. Further, the refinement of lignin and preparation of the lignosulfonates therefrom are expenses which cause the lignosulfonates to lose some of the cost advantage over that of the sulfonates commonly used in chemical floods.